This invention relates to a microwave power combining FET amplifier using a microwave integrated circuit.
Typical such amplifiers are disclosed, for example, in T. Takagi et al, "A 1.5 Watt 28 GHz Band FET Amplifier", 1984 IEEE MTT-S Digest, pages 227-228, and in S. D. Bingham et al, "A Miniaturized 6.5-16 GHz Monolithic Power Amplifier Module", IEEE 1985 Microwave and Millimeter-Wave Monolithic Circuits Symp. Digest, pages 38-41. The former uses two two-stage amplifiers operated in parallel and branch-line couplers for power splitting/combining, and the latter uses four monolithic two-stage amplifiers operated in parallel and inter-digital couplers for power splitting/combining.
FIG. 1 is a circuit diagram of a microwave power combining FET amplifier as shown, for example, in the above-mentioned 1984 IEEE MTT-S Digest, wherein reference numeral 1 designates an input terminal, 2 is an output terminal, 3a and 3b are pre-stage FETs, 4a and 4b are post-stage FETs, 5a and 5b are input matching circuits, 6a and 6b are interstage matching circuits, 7a and 7b are output matching circuits, 8 is a power splitter, and 9 is a power combiner. The power splitter 8 and power combiner 9 use branch-line type couplers or Wilkinson type couplers.
In operation, microwave power inputted from terminal 1 is split by the power splitter 8, and an "upper" portion of the divided power is supplied to the combiner 9 through the input matching circuit 5a, pre-stage FET 3a, interstage matching circuit 6a, post-stage FET 4a, and output matching circuit 7a. The "lower" portion of the power is similarly supplied to the combiner 9 through the companion components 5b, 3b, 6b, 4b, and 7b. The split microwave powers are matched by the input interstage, and output matching circuits, amplified by the preand post-stage FETs, recombined in the power combiner 9, and outputted at terminal 2.
The power splitter 8 and power combiner 9 have internal resistance isolation circuits for absorbing reflected microwave power which propagates in an odd mode due to portions of the divided signals having equal amplitudes and opposite phases caused by non-uniformities between the individual FETs 3a, 3b, 4a, and 4b. As a result, the amplifier shown in FIG. 1 can be operated stably.
To provide such resistance isolation, however, branch-line couplers or Wilkinson couplers are required. This complicates the structure of the amplifier and impedes miniaturization.